1. Field of the Invention
The present invention relates to an integrated semiconductor device containing a plurality of semiconductor elements and a method of manufacturing the integrated semiconductor device. In particular, the present invention relates to an integrated semiconductor device containing multiple types of semiconductor elements with respective breakdown-voltage performances and on-resistances different from each other.
2. Description of the Background Art
As for the conventional MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor), in order to fabricate an IC (Integrated Circuit) containing MOSFETs with different threshold voltages, the IC is structured by being provided with well regions of different impurity concentrations. Specifically, a well region of a MOSFET with a higher threshold voltage has a lower impurity concentration while a well region of a MOSFET with a lower threshold voltage has a higher impurity concentration (Japanese Patent Laying-Open No. 11-111855).
As for MOSFETs used for a liquid-crystal driver for example, a transistor processing a logic signal and a transistor processing an output signal should have different transistor breakdown-voltages respectively. In other words, a MOSFET processing the logic signal has a well region with a relatively high impurity concentration as the MOSFET is driven by a low voltage and should be small in size. On the other hand, a MOSFET processing the output signal has a well region with a relatively low impurity concentration as the MOSFET is driven by a high voltage and accordingly requires a high breakdown voltage (Japanese Patent Laying-Open Nos. 6-318561 and 11-238806).
For the above-discussed IC, wells of two different impurity concentrations respectively are formed by using an implantation mask having its parts with different opening ratios respectively to implant impurities into a semiconductor. After this, annealing is performed to diffuse the implanted impurities thereby producing a uniform concentration in each well. It is apparently seen that the well into which impurities are implanted through the part of the mask having a lower opening ratio has a lower impurity concentration.
The implantation mask having its parts with different opening ratios respectively can thus be used to change the amount of implanted impurities depending on the well to provide different impurity concentrations of the wells for respective semiconductor elements.
Here, what should be considered is the impurity concentration of the well, namely body, of the MOSFET, and relevant characteristics of the MOSFET are (b1) threshold voltage and (b2) breakdown voltage. More specifically, in order to produce MOSFETs that are different in threshold voltage or breakdown-voltage performance, respective concentrations of wells are made different.
It is still required, however, to reduce the on-resistance of each of high breakdown-voltage transistors included in an integrated semiconductor device. Even if the well or body of a high breakdown-voltage transistor is changed in impurity concentration, which has almost no influence on the on-resistance. Then, it has been desired that desirable characteristics of an integrated semiconductor device which has a plurality of high breakdown-voltage elements are ensured by setting both of an appropriate breakdown voltage and an appropriate on-resistance of each high breakdown-voltage transistor element.